1. Field of the Invention
This invention relates to nuclear magnetic resonant microscopy and more specifically to nuclear magnetic resonant microscopy employing a superconducting resonator for coupling signals to and from a speciment for examination.
2. Description of Related Art
In any imaging experiment, the signal-to-noise ratio (SNR) determines the limits of resolution. For whole body magnetic resonance imaging (MRI) at a field strength of 1.5 Tesla, it is the subject being imaged, i.e., the patient, who contributes most of the noise to the image. As the dimensions of the subject and RF coil are reduced, a point is reached at which the RF coil contributes more to the total noise than the subject. This point is dependent upon field strength, geometry, etc. and other factors described in Scaling Laws and Cryogenic Probes for NMR Microscopy by Black et al., pg. 1250, Proceedings of the 10th Annual Conference of the Soc. of Magn. Reson. in Med., held in San Francisco, Calif. Aug. 10-16, 1991. In the case of nuclear magnetic resonance microscopy (NMRM), it is the thermal or Johnson noise of the RF coil that dominates. Thus, it is the Johnson noise of the RF coil that currently limits the SNR of NMRM.
The theoretical underpinnings of Johnson noise are well established, as described in Thermal Agitation of Electricity in Conductors by J. B. Johnson, Physical Review, Jul. 1928. The two parameters that determine the magnitude of Johnson noise, and which are under the control of the experimenter, are resistance (R) of the RF coil, and temperature (T). There have been earlier attempts to cool conventional metal coils to reduce Johnson noise as described in A High-Resolution NMR Probe in Which the Coil and Preamplifier Are Cooled With Liquid Helium by Styles et al., J. Mag. Res., 60, pp. 397-404 (1984).
Reducing the resistance of the RF coil has been accomplished in the past by taking advantage of the decrease in resistance of normal metals (e.g., copper) with decreasing temperature. A much greater reduction is realized with the use of superconducting metals.
Bulk, polycrystalline Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 (YBCO) super-conductor has been used to produce a crude head coil resonator for whole body imaging as described in Improvements in High Temperature Superconductor Receiver Coils by Hall et al., pg. 725, Proceedings of the 10th Annual Conference of the Soc. of Magn. Reson. in Med., held in San Francisco, Calif. Aug. 10-16, 1991. The quality factor Q of this coil was not substantially better than a resonator employing copper. Also, this coil cannot properly function in high-field imaging.
Currently there is a need to reduce the noise in NMRM, and increase the signal-to-noise ratio in order to provide microscopic images.